In the related art, techniques of supplying power in a wireless manner by using electromagnetic induction or electromagnetic wave have been considered. Recently, techniques of supplying power in a wireless manner by using magnetic field resonance have been considered. The magnetic field resonance is a phenomenon where two resonating coils are coupled with each other through a magnetic field, so that energy transfer occurs. The magnetic field resonance is referred to as the resonance of a magnetic field.    Patent Document 1: Japanese Laid-open Patent Publication No. 2009-106136    Patent Document 2: Japanese National Publication of International Patent Application No. 2009-501510    Patent Document 3: Japanese National Publication of International Patent Application No. 2002-544756    Patent Document 4: Japanese Laid-open Patent Publication No. 2008-301918    Patent Document 5: Japanese Laid-open Patent Publication No. 2008-160312    Patent Document 6: Japanese Laid-open Patent Publication No. 2006-230129
Energy is transferred between coils, and a load is connected to the coil at an energy extraction side, so that power can be supplied to the load. Power supply efficiency depends on impedance of the load.
In the case where a battery is connected as the load, the impedance of the load is sequentially changed according to a charging state of the battery. Therefore, in the related art, there is a situation that the power supply efficiency is deteriorated in the period from the discharged state of the battery to the fully-charged state thereof.